Removal of filter cake from earth bores



Patented Aug. 12, 1947 UNITED REMOVAL OF FILTER CAKE FROM EARTH 'BORES Donald 0. Bond, Northbrook, Ill., assignor to The Pure Oil Company, Chicago, 111., a corporation of Ohio 6 Claims.

This invention relates to the removal of filter cake from the walls of earth bores and more particularly to the removal of filter cake/resulting from the drilling of oil or gas wells in whichis used a basic dye-starch positive colloid drilling mud. This application is a continuation of application Ser. No. 514,967, filed Dec, 20, 1943.

In Patent No. 2,273,925 there is disclosed a process of drilling wells in which is used a, basic dye, positive colloid drilling mud containing starch, wheat gluten, and or acid casein. This invention is directed to removing from the Well-bore the filter cake resulting from the use of drilling muds disclosed in the aforesaid patent. It is necessary to remove the filter cake from the portion of the wall passing through the producing formation in order to permit free passage of oil from the producing formation into the well-bore.

As disclosed in Patent No. 2,273,925 the mud contains a basic dye such as methyl violet, bentonite, water, an anti-gas-cutting agent such as diglycol laurate, a stabilizing agent selected from the group consisting of starch, wheat gluten and acid casein, barytes as weighting materiaLand sufiicient hydrochloric acid, when necessary, to adjust the pH of the solution to a desirel level below about 7.0 to 7.5. Bentonite is used in the amount of l or 2% by weight based on the unweighted mud. The stabilizing agent may be present in amounts of from 1 to by weight. The basic dye is present in amount sufiicient to produce positive charges on the mineral particles dispersed in the water. The amount of barytes is sufi'icient to give the desired specific gravity to the mud. In the examples given in the patent 350 grams of barytes are added to 500 grams of the remaining constituents of the mud to obtain a specific gravity of 1.5.

I have discovered that the filter cake resulting from use of drilling muds of the naturedescribed in the aforesaid patent can be readily disintegrated by bringing in contact with the filter cake a strong mineral acid of proper concentration. As acids which may be used to disintegrate the filter cake, may be mentioned hydrochloric, sulfuric, nitric, hydrofluoric and fluoboric acid, although I prefer to use hydrochloric, sulfuric or fiuoboric acids for the reason that these acids can be inhibited against corrosion of the metal casing in the well-bore and do not form objectionable products by reaction with the earth in the Wellbore, which clog the producing formation.

The minimum concentration of acid which is required todisintegrate the filter cake is specific for each acid.

Filter cake resulting fromuse of basic dye-starch positive colloid drilling mud requires hydrochloric acid solution having a concentration greater than 3 normal and preferably a concentration of about 6 normal. Concentrations below 3 normalhave very little effect-on the disintegration of the filter cake. Sulfuric acid should be used in concentrations of approximately 12 normal or greaterito disintegrate the filtercake. Nitric acid should be used in a concentration of not less than approximately 2 normal. Hydrofluoric acid should be used in concentrations of not less than approximately 20 normal and fluoboricacid should .be used in concentrations of .not less than about 2 normal.

The concentration of the acid which is,necessary to disintegrate the filter cakeresulting from drilling muds of the type disclosedin theaforesaid Patent No. 2,273,925 is less than that which is required to disintegrate filter cake resulting from the .use of starch-containing, negatively chargedcolloid'mud. Use of lower concentration acids is advantageous in thatit cuts down corrosion of metal casing ,in the well-bore.

In order to demonstratetheinvention, the following tests were made. .Suspensionsof wheat flour in plain Water and in methyl violet solution were prepared by allowing the mixtureof wheat flour and water, .or solution, to stand at F. for two hours, with occasional stirring. The suspensions were cooled to room temperature and were then allowed to filterthrough a filter paper on a Buchnerlfunnel 3.5 inches in diameter under a pressure of 650 mm. of ,mercuryforone hour, until a. filter cake was built upon the filter paper. The liquid remaining .in the filter was poured oil and the filter cake Was washed with water. The filter cake was then tested by first filtering water therethrough for one-half hour to determine the permeability of the cake to water. The water was then poured off the filter cake and various strong mineral acids of difierent concentrations were then tested by allowing each to filter through a fresh, washed, tested filter cake and the quantity of filtrate in a given time was determined. The results of the tests are given in the following table.

4 I claim: 1. The method of removing from the wall of an Table Composition of Mixture Filtered, Per cent by C H 0c. Acid Through Filter Cake in- W ht Ce 2 mg Through Test No. Filtrate Acld Used in 1 Hr F lter ake Methyl Wheat W t m /2 1 2 4 5 e 19 1 5 2o 30 Violet Flour er Min. Mln M111 Min Min. Mm Mm. Mm Min 3 87 3 1 0 0 3 10 87 3 1 O 0 3 10 87 2 1 0 0 3 1O 87 2 1 0 O 3 10 87 2 O. 5 0 0 3 1O 87 3. 5 1 O 0 3 10 87 3. 5 1 0 0 3 10 87 2 l 0 0 3 10 87 2. 5 0. 5 0 0 3 10 87 2. 5 1 0 0 3 10 87 1 1 0 2O 3 1O 87 l. 5 0. 5 0 0 3 10 87 3 1. 5 0 0 3 10 87 3. 5 1 O 0 3 10 87 2. 5 1. 5 0 0 3 10 87 3. 5 1 0 0 10 90 l. 5 l. 5 0.5N HNOs.-.-. 0 0. 5

From the table it is apparent that filter cake formed from the mixture containing 3% methyl violet and 10% wheat flour was decomposed by acid of lower concentration than filter cake formed from the suspension containing no methyl violet. It is also apparent that fluoboric acid was more effective than the other acids when used in low concentration. The quantity of methyl violet in this suspension imparted a positive charge to the suspension and was sufiicient to impart a positive charge to a drilling mud containing bentonite or other mineral colloid material,

Although our invention is applicable generally to basic dye positive colloid drilling muds of the type disclosed in Patent No. 2,273,925, it is particularly applicable to those muds prepared from starch, basic dye and mineral colloid particles, such as bentonite, suspended in water.

Although methyl violet was used as the basic' dye for the purpose of the tests recorded in the foregoing table, it is to be understood that the invention is not limited to this particular dye, but is directed to basic dyes in general, including methylene blue, methylene green, methylene grey, methyl violet, Bismarck Brown R, Rhodamine B, acrifiavine, chrysoidin Y and chrysoidin R. It should be furthe understood that the invention is directed to the use of starches in general and not limited to the particular starch (wheat flour) used in the tests.

It will be seen, therefore, that I have discovered a method of disintegrating filter cake resulting from the use of basic dye, positively charged, colloid, drilling muds containing a sealing agent, such as starch, and that I have not only discovered that strong acids of specific concentrations are required to disintegrate the filter cake, but that the concentration of acid is far lower than that required by starch-containing, negative colloid, drilling muds.

earth bore filter cake deposited from basic dye, positively charged, colloid, drilling mud composed substantially only of water, basic dye, an antigas-cutting agent, bentonite in the amount of approximately 1 to 2% by weight based on the unweighted mud, sufiicient barytes to weight the mud to desired specific gravity and a material selected from the group consisting of starch, wheat gluten and acid casein, comprising introducing fiuoboric acid of sufiicient concentration to attack said filter cake into contact therewith.

2. The method in accordance with claim 1 in which the fluoboric acid has a concentration not less than approximately 2 normal.

3. The method in accordance with claim 1 in which the fiuoboric acid has a concentration of 138-55 normal.

4. Method in accordance with claim 1 in which the material is starch.

5. Method in accordance with claim 1 in which the basic dye is methyl violet.

6. Method in accordance With claim 5 in which the anti-gas-cutting agent is diglycol laurate.

DONALD C. BOND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Lehnhard, Mud Acid; article in the Petroleum Engineer Reference Annual, 1943, pages 82, 85, 86, 88, 90, 92, and 98. 

